One type of device for clamping weft threads in a warp knitting machine is disclosed in DE-OS No. 20 12 114. In this type of device the clamping part includes a pressure wheel which is formed of elastic material and its circumference engages the weft thread hooks to hold in position the weft threads placed around the hooks. It is necessary to retain the weft threads against the hooks because weft thread waste pieces extend around the hooks after the weft threads have been incorporated and cut off. Unless the weft thread waste pieces are retained on the weft thread hooks, the weft threads will not remain under tension and may become loose. The pressure wheel is positioned at the same position as the weft thread cutting location and is rotated by the weft thread hooks as they move forwardly so that the inward pressure of the pressure wheel engages and presses against at least two hooks during its rotation, because of the diameter and elasticity of the pressure wheel. The pressure wheel thus acts only with a relatively short clamping zone as the longitudinal conveyors are advanced so that the clamping of the weft threads by the pressure wheel takes place over a short weft thread racking zone as the weft threads extend back and forth between the opposite longitudinal conveyors. As a greater number of weft threads are placed on the hooks by the thread guide, the weft threads will be laid on top of one another in certain hooks and must be clamped in this position by the pressure wheel. This may lead to the release of certain of the weft yarns, particularly when very smooth and slick yarns are employed, so that the tension in the weft threads can be released and allow the yarns to slip back in relation to the hooks and the pressure wheel.
In an attempt to overcome this problem of the short clamping zone provided by the pressure wheel, it has been proposed that the pressure wheel be elongated to form a kind of revolving caterpillar so that a pressure belt is pressed against the hooks by sequentially arranged pressure rolls. With this arrangement, the weft thread clamping zone is lengthened, in accordance with the length of the caterpillar pressure belt. In this arrangement, the pressure belt is moved along by friction against the hooks and the weft threads, in a similar manner as described above for the pressure wheel. This pressure belt arrangement also does not always operate satisfactorily, especially when weft thread arrays including a large number of threads, for example 20 weft threads, are fed to the hooks so that the hooks in the center of the array have large bunches of threads being pressed against the hooks by the pressure belt. The thickness of the bunch of weft threads is not evenly distributed throughout the weft thread waste pieces, but tapers evenly from the center to both sides and thus has to be compensated by the caterpillar pressure belt. It has been found that an even contact pressure function of the caterpillar pressure belt cannot be achieved across a large clamping zone so that, especially with warp threads having a low frictional contact with the hooks, the same problems occur as with the contact pressure wheel described above. Also, the contact pressure wheel and the contact pressure belt are each subject to a high degree of wear.